Nuclear reactors have required and have been provided with cooling means since the early days of nuclear reactor development. Various coolants have been tried.
In the past liquid metal nuclear reactors have been made which have been cooled with mercury, liquid sodium or a liquid sodium-potassium mixture.
There are two basic types of liquid metal reactors which have been under development for commercial nuclear power applications. These reactors have been described as loop reactors such as the Fast Flux Test Reactor and the Clinch River Breeder Reactor in the United States or the pool type reactor such as the Phenix in France. These reactors have used liquid metal sodium as the primary coolant.
In the loop reactor, the core, pumps and intermediate heat exchangers are located in separate vessels and connected with pipes. The sodium is pumped from the core to the heat exchanger and back to the core. There is no pool of liquid metal in this reactor as there is in a pool reactor.
The pool reactor comprises a large vessel or pot containing liquid sodium in which the reactor core, intermediate heat exchangers and the primary circulating pumps are located. In this configuration, the cool liquid sodium is pumped to the core from which it flows into a pool of hot sodium and then through the heat exchanger and is discharged into a pool of cool sodium, prior to entry to the pumps.
In the prior art, alternate designs of the loop reactors have been described wherein the intermediate heat exchanger and the pump have been housed in an auxilliary vessel which is connected to the main reactor vessel by a duct having a coaxial pipe.
The prior art loop reactors have required complex pipe arrangements including snubbers, hangers, heating systems, insulation, inspection capability means and usually a check valve and flow measuring device. The entire primary system is enclosed within an inert atmosphere and a steel lined concrete cell compartment. Guard vessels or some other means such as siphon breakers are provided to insure a safe minimum level of sodium within the reactor vessel in the event of a leak in the piping system.
The prior art pool reactors have been difficult to fabricate and have presented engineering difficulties due to the relatively confined area of the main reactor vessel which contains the reactor core, intermediate heat exchangers and pumps as well as reactor core shielding. The size of these vessels requires a complex fabrication and assembly process with the accompanying quality control problems. The relatively crowded interior of the pool reactor provides little design flexibility and may necessitate compromises in the design of the pump, heat exchanger and associated structures.
The amount of primary coolant embodied by the pool concept is usually larger than the loop plant thus providing an added margin for safety in the event all decay heat removal systems fail. For Superphenix I, a separate containment structure is provided for the atmosphere above the vessel head because safety authorities have postulated that conditions may occur which can cause the primary coolant to be ejected through the seals of the vessel head. A steel cover may be provided above the vessel to act as a containment barrier but such a cover is difficult to provide because of the large size of the head, the many penetrations required for completing the circuits of the secondary coolant and the need to remove, for maintenance and repair, the primary pump or the primary heat exchanger. The pool type of design does not require separate primary cells, a primary cell inerting and cooling system and primary piping hangers or snubbers. In addition the pool reactor due to the multiplicity of pumps hydraulically connected to a common pool has the advantage of not requiring fast acting check valves in the primary circuit and the operational flexibility of not requiring immediate trip of the pumps during a reactor shutdown.